This invention relates to an integrated circuit comprising a gas sensor.
Nowadays, integrated circuits may comprise a plethora of different sensors, such as ambient light (AL) sensors, temperature (T) sensors, gas sensors, relative humidity (RH) sensors, specific analyte detection sensors, and so on.
Integrated circuits of this kind have a wide range of applications. For example, they can be used in the field of supply chain management to track and monitor the freshness of food and beverages. They can also be used as environmental sensors, for example as part of a heating, ventilation and air conditioning (HVAC) system in an automobile or in a building (e.g. a Smart Building). Additional applications include those in agricultural (e.g. the sensing of environmental conditions in greenhouses) or in medical fields. Their provision in mobile communications devices such as mobile telephones, tablets or laptops can also enable a wide range of further applications that require measurements of local environmental factors.
The provision of sensors in integrated circuits of this kind allows devices to be produced that have a small form factor. For example, due to their small form factor, integrated circuits incorporating one or more sensors can be included in Radio Frequency Identification (RFID) tags, allowing for easy programming and readout.
Moreover, it allows large numbers of sensors to be manufactured cheaply, using established semiconductor processing techniques.
FIG. 1 illustrates an example of an integrated circuit comprising a gas sensor. The integrated circuit includes a semiconductor substrate 2. The substrate typically comprises various components and circuitry, including, for example CMOS transistors and/or other devices. These components can be formed in the substrate 2 using standard front end of line (FEOL) processes.
The integrated circuit also includes a metallization stack 4. Metallization stacks, which are well known in the art of semiconductor device manufacturing, typically include a plurality of dielectric layers containing patterned metal features. These metal features form interconnections for connecting together the various components of the integrated circuit formed in the substrate.
The integrated circuit further includes a passivation stack 6. The passivation stack 6 can include one or more layers forming a protective covering for the metallization stack 4 against, for example, oxidation of the metal features therein.
In the example shown in FIG. 1, the gas sensor includes a pair of capacitor electrodes 10. The electrodes 10, shown in cross section in FIG. 1, are arranged as a set of interdigitated fingers. For further examples of sensors using such an electrode configuration see U.S. Pat. No. 7,222,531, U.S. Pat. No. 6,690,569, U.S. Pat. No. 4,429,343 and U.S. Pat. No. 6,222,376). The electrodes are electrically conductive, and can comprise a metal such as aluminium (Al) or tungsten (W). The electrodes 10 are formed directly on the passivation stack 6 using the same kind of back end of line (BEOL) processes (deposition, lithography, etching) that are used to form the metal features in the metallization stack 4.
The gas sensor also includes a gas sensitive layer 12. The gas sensitive layer 12 has a dielectric constant that is sensitive to a gas to be sensed. The type of gas sensitive material that is used can thus be chosen in accordance with the target gas. In operation, the dielectric constant of the gas sensitive layer 12 varies in accordance with the levels of the target gas that are present in the vicinity of the sensor. These variations in dielectric constant can be detected as changes in the capacitance of the capacitor electrodes 12. Hence, the target gas can be detected.
Since the capacitor electrodes 10 are provided outside the passivation stack 6, they may be susceptible to corrosion. Because of this, the gas sensor further includes a protective layer 8, which acts to prevent corrosion of the electrodes 10. The protective layer 10 comprises an oxide such as TaO5, which is electrically insulating.